Thin Al/Zn-based alloy coatings (2-100 μm) are often applied to the surfaces of steel strip to provide protection against atmospheric corrosion.
These alloy coatings are generally, but not exclusively, coatings of alloys of elements Al, Zn, Mg, Si, Fe, Mn, Ni, Sn and other elements such as V, Sr, Ca, Sb in small amounts.
These alloy coatings are generally, but not exclusively, applied to steel strip by hot dip coating strip by passing strip through a bath of molten alloy. The steel strip is typically, but not necessarily exclusively, heated prior to dipping to promote bonding of the alloy to the strip substrate. The alloy subsequently solidifies on the strip and forms a solidified alloy coating as the strip emerges from the molten bath.
The cooling rate of the alloy coating is relatively low, typically less than 100° C./s. The cooling rate is restricted by the thermal mass of the strip and by impact damage of the hot, soft coating by cooling media.
The low cooling rate means that the microstructure of the Al/Zn-based alloy is a relatively coarse dendritic and/or lamellar structure comprising a mixture of phases of different compositions.
Other known means of forming Al/Zn-based alloy coatings onto steel strip produce molten alloy coatings that solidify in different manners to hot-dip coatings. However, the Al/Zn-based alloys of the coatings still exist as relatively coarse mixtures of phases of different compositions.